They are. It's not a cheat, it predicting and calculations on the part. Which is programed in the game, which makes it perfectly fine for VGC. If they get a bit curious, just say you got really lucky. Marriland knows about the stuff, as does Mike L, I would believe, so there is no problem in that.
Also to add, some kid had his whole team shiny, and passing the hack check. They were most likely RNG'd by his parents or so, if they are very much into the game.
I've only gotten shinys on the part, and it takes me forever to get right. Only one, if I could add. But, on the point, they are all legal as it's just you predicting the right time to get to what is needed.